It is standard in the field of integrated circuit memories to use a sense amplifier that converts a small voltage swing on a bit line to a logic level signal for further processing. These sense amplifiers compare the voltage on the bit line with a reference voltage level in order to determine if the cell is storing a logical one or zero.
A simple form of voltage reference used in the prior art is a resistor divider chain between the power supply voltage and ground, but these dividers set up a low impedance path to ground that draws an amount of current that is unacceptably large for modern circuits.
The prior art has devoted considerable effort to making a reference voltage stable against variation in the power supply voltage, but when that is done in a situation in which the logical one is not the power supply voltage and/or the logical zero is not ground, the reference voltage may drift above or below the logical one or logical zero voltage values, respectively. It would be very useful in such memories, to have a trip voltage that would vary with the power supply voltage in the same manner as the logical one or logical zero voltage levels vary.